The description of the prior art will be primarily based on the list of references presented at the end of this section. Since the present invention is primarily related to the lithium ion battery and supercapacitor, the discussion on the prior art will be divided into two parts:
Part A. Secondary Battery (Particularly, Lithium Ion Battery)
Concerns over the safety of earlier lithium secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials as an anode. There are three fundamentally distinct types of carbonaceous anode materials: (a) graphite, (b) amorphous carbon, and (c) graphitized carbon.
The first type of carbonaceous material includes primarily natural graphite and synthetic graphite (or artificial graphite, such as highly oriented pyrolitic graphite, HOPG) that can be intercalated with lithium and the resulting graphite intercalation compound (GIC) may be expressed as LixC6, where x is typically less than 1. In order to minimize the loss in energy density due to the replacement of lithium metal with the GIC, x in LixC6 must be maximized and the irreversible capacity loss Qir in the first charge of the battery must be minimized. The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal is generally believed to occur in a graphite intercalation compound represented by LixC6 (x=1), corresponding to a theoretical specific capacity of 372 mAh/g.
Carbon anodes can have a long cycle life due to the presence of a protective surface-electrolyte interface layer (SEI), which results from the reaction between lithium and the electrolyte (or between lithium and the anode surface/edge atoms or functional groups) during the first several charge-discharge cycles. The lithium in this reaction comes from some of the lithium ions originally intended for the charge transfer purpose. As the SEI is formed, the lithium ions become part of the inert SEI layer and become irreversible, i.e. they can no longer be the active element for charge transfer. Therefore, it is desirable to use a minimum amount of lithium for the formation of an effective SEI layer. In addition to SEI formation, Qir has been attributed to graphite exfoliation caused by electrolyte/solvent co-intercalation and other side reactions [Refs. 1-4].
The second type of anode carbonaceous material is amorphous carbon, which contains no or very little micro- or nano-crystallites. This type includes the so-called “soft carbon” and “hard carbon.” The soft carbon is a carbon material that can be readily graphitized at a temperature of 2,500° C. or higher. The hard carbon is a carbon material that cannot be graphitized even at a temperature higher than 2,500° C. In actuality, however, the so-called “amorphous carbons” commonly used as anode active materials are typically not purely amorphous, but contain some small amount of micro- or nano-crystallites. A crystallite is composed of a small number of graphene sheets (basal planes) that are stacked and bonded together by weak van der Waals forces. The number of graphene sheets varies between one and several hundreds, giving rise to a c-directional dimension (thickness Lc) of typically 0.34 nm to 100 nm. The length or width (La) of these crystallites is typically between tens of nanometers to microns.
Among this class of carbon materials, soft and hard carbons made by low-temperature pyrolysis (550-1,000° C.) exhibit a reversible capacity of 400-800 mAh/g in the 0-2.5 V range [Refs. 1-3]. Dahn et al. have made the so-called house-of-cards carbonaceous material with enhanced capacities approaching 700 mAh/g [Refs. 1,2]. Tarascon's research group obtained enhanced capacities of up to 700 mAh/g by milling graphite, coke, or carbon fibers [Ref. 3]. Dahn et al. explained the origin of the extra capacity with the assumption that in disordered carbon containing some dispersed graphene sheets (referred to as house-of-cards materials), lithium ions are adsorbed on two sides of a single graphene sheet [Refs. 1,2]. It was also proposed that Li readily bonded to a proton-passivated carbon, resulting in a series of edge-oriented Li—C—H bonds. This provides an additional source of Li+ in some disordered carbons [Ref. 5]. Other researchers suggested the formation of Li metal mono-layers on the outer graphene sheets [Ref. 6] of graphite nano-crystallites. The amorphous carbons of Dahn et al. were prepared by pyrolyzing epoxy resins and may be more correctly referred to as polymeric carbons. Polymeric carbon-based anode materials were also studied by Zhang, et al. [Ref. 16] and Liu, et al. [Ref. 17].
The following mechanisms for the extra capacity over the theoretical value of 372 mAh/g have been proposed [Ref. 4]: (i) lithium can occupy nearest neighbor sites; (ii) insertion of lithium species into nano-scaled cavities; (iii) lithium may be adsorbed on both sides of single layer sheets in very disordered carbons containing large fractions of single graphene sheets (like the structure of a house of cards) [Refs. 1,2]; (iv) correlation of H/C ratio with excess capacity led to a suggestion that lithium may be bound somehow in the vicinity of the hydrogen atoms (possible formation of multi-layers of lithium on the external graphene planes of each crystallite in disordered carbons) [Ref. 6]; and (vi) accommodation of lithium in the zigzag and armchair sites [Ref. 4].
Despite exhibiting a high capacity, an amorphous carbon has a low electrical conductivity (high charge transfer resistance) and, hence, resulting in a high polarization or internal power loss. Conventional amorphous carbon-based anode materials also tend to give rise to a high irreversible capacity.
The third type of anode carbonaceous material, graphitized carbon, includes meso-carbon microbeads (MCMBs) and graphitized carbon fibers (or, simply, graphite fibers). MCMBs are usually obtained from a petroleum heavy oil or pitch, coal tar pitch, or polycyclic aromatic hydrocarbon material. When such a precursor pitch material is carbonized by heat treatment at 400° to 500°, micro-crystals called mesophase micro-spheres are formed in a non-crystalline pitch matrix. These mesophase micro-spheres, after being isolated from the pitch matrix (pitch matrix being soluble in selected solvents), are often referred to as meso-carbon microbeads (MCMBs). The MCMBs may be subjected to a further heat treatment at a temperature in the range of 500° C. and 3,000° C. In order to obtain a stably reversible capacity in an anode, commercially available MCMBs are obtained from heat-treating mesophase carbon spheres at a temperature typically above 2,000° C. and more typically above 2,500° C. for an extended period of time. Graphitized carbons have several drawbacks:                (1) Due to such time-consuming and energy-intensive procedures, MCMBs have been extremely expensive. Likewise, the production of all types of graphite fibers (vapor-grown, rayon-based, pitch based, and polyacrylonitrile-based) is also tedious and energy-intensive and the products are very expensive.        (2) The production of meso-carbon microbeads having a very small diameter, particularly 5 μm or less has been difficult. When the concentration of optically anisotropic small spheres (meso-phase spheres) increases, the small spheres tend to coalesce and precipitate to produce bulk mesophase and separation of small spheres becomes difficult. This is likely the reason why MCMBs with bead size less than 5 μm are not commercially available. Smaller anode active material particles are essential to high-rate capacity of a lithium ion battery.        (3) Furthermore, both MCMBs and graphite fibers give rise to an anode capacity of typically lower than 350 mAh/g and more typically lower than 320 mAh/g.        
Therefore, an urgent need exists for a carbon/graphite-based anode material that has the following highly desirable features: low cost, mass-producibility, high reversible capacity, low irreversible capacity, small particle sizes (for high-rate capacity), compatibility with commonly used electrolytes, and long charge-discharge cycle life.
Part B. Supercapacitor
Electrochemical capacitors (ECs), also known as ultracapacitors or supercapacitors, are being considered for uses in hybrid electric vehicles (EVs) where they can supplement a battery used in an electric car to provide bursts of power needed for rapid acceleration, the biggest technical hurdle to making battery-powered cars commercially viable. A battery would still be used for cruising, but supercapacitors (with their ability to release energy much more quickly than batteries) would kick in whenever the car needs to accelerate for merging, passing, emergency maneuvers, and hill climbing. The EC must also store sufficient energy to provide an acceptable driving range. To be cost- and weight-effective compared to additional battery capacity they must combine adequate specific energy and specific power with long cycle life, and meet cost targets as well. Specifically, it must store about 400 Wh of energy, be able to deliver about 40 kW of power for about 10 seconds, and provide high cycle-life (>100,000 cycles).
ECs are also gaining acceptance in the electronics industry as system designers become familiar with their attributes and benefits. ECs were originally developed to provide large bursts of driving energy for orbital lasers. In complementary metal oxide semiconductor (CMOS) memory backup applications, for instance, a one-Farad EC having a volume of only one-half cubic inch can replace nickel-cadmium or lithium batteries and provide backup power for months. For a given applied voltage, the stored energy in an EC associated with a given charge is half that storable in a corresponding battery system for passage of the same charge. Nevertheless, ECs are extremely attractive power sources. Compared with batteries, they require no maintenance, offer much higher cycle-life, require a very simple charging circuit, experience no “memory effect,” and are generally much safer. Physical rather than chemical energy storage is the key reason for their safe operation and extraordinarily high cycle-life. Perhaps most importantly, capacitors offer higher power density than batteries.
The high volumetric capacitance density of an EC (10 to 100 times greater than conventional capacitors) derives from using porous electrodes to create a large effective “plate area” and from storing energy in the diffuse double layer. This double layer, created naturally at a solid-electrolyte interface when voltage is imposed, has a thickness of only about 1-2 nm, thus forming an extremely small effective “plate separation.” In some ECs, stored energy is further augmented by pseudo-capacitance effects, occurring again at the solid-electrolyte interface due to electrochemical phenomena such as the redox charge transfer. The double layer capacitor is based on a high surface area electrode material, such as activated carbon, immersed in an electrolyte. A polarized double layer is formed at electrode-electrolyte interfaces providing high capacitance.
Experience with ECs based on activated carbon electrodes shows that the experimentally measured capacitance is always much lower than the geometrical capacitance calculated from the measured surface area and the width of the dipole layer. For very high surface area carbons, typically only about ten percent of the “theoretical” capacitance was observed. This disappointing performance is believed to be related to the presence of micro-pores and ascribed to inaccessibility of some pores by the electrolyte, wetting deficiencies, and/or the inability of a double layer to form successfully in pores in which the oppositely charged surfaces are less than about 2 nm apart. In activated carbons, depending on the source of the carbon and the heat treatment temperature, a surprisingly high amount of surface can be in the form of such inaccessible micro-pores.
It would be desirable to produce an EC that exhibits greater geometrical capacitance using a carbon based electrode having a high accessible surface area, high porosity, and reduced or no micro-pores. It would be further advantageous to develop carbon-based nano-structures that are conducive to the occurrence of pseudo-capacitance effects such as the redox charge transfer.
In this context, carbon nanotubes (CNTs) are of great interest. CNTs are nanometer-scale sized tube-shaped molecules having the structure of a graphite molecule rolled into a rube. A nanotube can be single-walled or multi-walled, dependent upon conditions of preparation. Carbon nanotubes typically are electrically conductive and mechanically strong and stiff along their length. Nanotubes typically also have a relatively high aspect ratio (length/diameter ratio). Due to these properties, the use of CNTs as reinforcements in composite materials for both structural and functional applications would be advantageous. In particular, CNTs are being studied for electrochemical supercapacitor electrodes due to their unique properties and structure, which include high surface area, high conductivity, and chemical stability. Capacitance values from 20 to 180 F/g have been reported, depending on CNT purity and electrolyte, as well as on specimen treatment such as CO2 physical activation, KOH chemical activation, or exposure to nitric acid, fluorine, or ammonia plasma. Carbon nano-fibers (CNFs) and graphitic nano-fibers (GNFs), two thicker-diameter cousins of CNTs, have also been investigated as potential EC electrode materials.
Conducting polymers, such as polyacetylene, polypyrrole, polyaniline, polythiophene, and their derivatives, are also common electrode materials for supercapacitors. The modification of CNTs with conducting polymers is one way to increase the capacitance of the composite resulting from redox contribution of the conducting polymers. In the CNT/conducting polymer composite, CNTs are electron acceptors while the conducting polymer serves as an electron donor. A charge transfer complex is formed between CNTs in their ground state and aniline monomer. A number of studies on CNT/conducting polymer composites for electrochemical capacitor applications have been reported: e.g., [Refs. 21-28] are related to CNT-, CNF-, or GNF-based EC electrodes.
However, there are several drawbacks associated with carbon nano-tubes or nano-fibers for EC electrode applications. First, both nano-tubes and nano-fibers are extremely expensive. Second, both materials tend to form a tangled mess resembling a hairball, which is difficult to work with. For CNTs, the interior surface is not accessible by electrolyte and, hence, not capable of developing double-layer charges. These and other difficulties have limited efforts toward commercialization of supercapacitors containing nano-tube or nano-fiber based electrodes.
As a less expensive material, macroscopic scale flexible graphite sheet has been used in an integrated electrode/current collector for EC applications, wherein the flexible graphite sheet is used as a substrate to support thereon an electrode active material (e.g., activated carbon particles) [Refs. 29-31]. Actually, these carbon particles are embedded on the surface or into the bulk of a flexible graphite sheet. The “flexible graphite” is typically obtained by first treating natural graphite particles with an intercalating agent (intercalant) that penetrates into the inter-planar spacings of the graphite crystals to form a graphite intercalated compound (GIC). The GIC is then exposed to a thermal shock, up to a temperature of typically 800-1,100° C. to expand the intercalated particles by typically 80-300 times in the direction perpendicular to the graphene layers (basal planes) of a graphite crystal structure. The resulting expanded or exfoliated graphite particles are vermiform in appearance and are, therefore, commonly referred to as graphite worms. Hereinafter, the term “exfoliated graphite” will be used interchangeably with the term “expanded graphite.” The worms may be re-compressed together into flexible sheets which can be formed and cut into various shapes. These thin sheets (foils or films) are commonly referred to as flexible graphite. Flexible graphite can be wound up on a drum to form a roll of thin film, just like a roll of thin plastic film or paper. The flexibility or compressibility of flexible graphite or exfoliated graphite enables the hard solid carbon particles to be embedded into the flexible graphite sheet when solid carbon particles and exfoliated graphite are combined and calendared, roll-pressed, or embossed together. However, such a combined electrode/current collector as disclosed in [Refs. 29-31] has several major shortcomings:    (1) The exfoliated graphite or flexible graphite sheet cited in these patents is a passive material that is used solely as a substrate or binder material to hold the electrode active material together for forming an integral member (electrode/current collector). The flexible graphite or exfoliated graphite itself is not used as an electrode active material, i.e., it does not provide the diffuse double layer charges and, hence, does not contribute to the double layer capacitance.    (2) In order for a flexible graphite sheet or exfoliated graphite particles to hold activated carbon particles together, the total amount of exfoliated graphite must be at least 50% by volume or more. Individual graphite particles are a solid, not a liquid adhesive. Although exfoliated graphite particles themselves can be re-compressed together to form a cohered body, the resulting flexible graphite sheet is normally very fragile. When a large amount of exfoliated graphite is used, the relative proportion of the electrode active material (the material that actually contributes to double layer capacitance) is small. Consequently, the effective energy density of the resulting supercapacitor is significantly curtailed.    (3) By embedding activated carbon particles into a flexible graphite sheet or mixing activated carbon particles with exfoliated graphite particles, one tends to seal off the pores of activated carbon particles that have surface openings supposedly functioning to accommodate the liquid electrolyte. Mixing or embedding significantly reduces the amount of carbon particle pores that are designed to be accessible by liquid electrolyte, thereby reducing the effective electrolyte-electrode interface areas where double layer charges can be formed.    (4) The activated carbon particles utilized by Reynolds, et al. [Refs. 9-11] were typically in the range of 600 μm and 900 μm. They were too big to penetrate the inter-layer spaces (<2.8 nm within an inter-planar spacing of 0.335 nm) between two graphene planes of un-expanded graphite crystallites. They were also too big to penetrate the space (typically <10 μm) between graphite flakes (each flake comprising a multiplicity of graphene sheets bonded by van der Waal's forces). With a maximum average expansion ratio of 300, the original inter-planar spacing of 0.335 nm would become at most 100 nm on average. In rare cases, there could be some pores as large as 10 μm, but these pores are still too small to accept activated solid carbon particles. In actuality, the activated carbon particles are simply squeezed by and held in place between clusters of expanded graphite flakes. Of course, such a configuration is advantageous in that it provides a substrate with good electrical conductivity and this substrate functions as a current collector as well.
Instead of trying to develop much lower-cost processes for making CNTs, the applicants and co-workers have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but are more readily available and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called nano-sized graphene plates (NGPs). The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. In practice, NGPs are obtained from a precursor material, such as minute graphite particles, using a low-cost process, but not via flattening of CNTs. One of the cost-effective processes is exfoliation of graphite to produce graphite worms of loosely connected flakes, followed by separation of these flakes into isolated (unconnected) graphene platelets using mechanical means (air jet milling, rotating-blade shearing, etc). These nano materials could potentially become cost-effective substitutes for CNTs or other types of nano-rods for various scientific and engineering applications. These diligent efforts have led to the following patent applications [Refs. 32-40]:
For instance, Jang, et al. [Ref. 33] disclosed a process to readily produce NGPs in large quantities. The process includes the following procedures: (1) providing a graphite powder containing fine graphite particles preferably with at least one dimension smaller than 200 μm (most preferably smaller than 1 μm); (2) exfoliating the graphite crystallites in these particles in such a manner that at least two graphene planes are fully separated from each other, and (3) mechanical attrition (e.g., ball milling) of the exfoliated particles to become nano-scaled, resulting in the formation of NGPs with platelet thickness smaller than 100 nm. The starting powder type and size, exfoliation conditions (e.g., intercalation chemical type and concentration, temperature cycles, and the mechanical attrition conditions (e.g., ball milling time and intensity)) can be varied to generate, by design, various NGP materials with a wide range of graphene plate thickness, width and length values. We have successfully prepared NGPs with an average length in the range of 1 to 20 μm). However, the length or width can be smaller than 500 nm and, in several cases, smaller than 100 nm. Ball milling is known to be an effective process for mass-producing ultra-fine powder particles. The processing ease and the wide property ranges that can be achieved with NGP materials make them promising candidates for many important engineering applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGPs will be available at much lower costs and in larger quantities.
After an extensive and in-depth study of the electrochemical response of NGPs and their composites, we have found that a certain class of meso-porous composites containing NGPs as electrode ingredients exhibit superior charge double layer-type supercapacitance and redox charge transfer-type pseudo-capacitance. Some preferred compositions were described in an earlier application [40]. These electrode materials were made by simply bonding NGPs together with a binder material to form a flat sheet, layer, or plate as an electrode.
By contrast, the present invention provides a graphene nanocomposite composition that is made first by combining NGPs with a first binder or matrix material to form micron-sized solid particles using, for instance, an atomization or aerosol formation technique. These nanocomposite solid particles are substantially spherical or ellipsoidal in shape and are of approximately 1-20 μm in size (preferably smaller than 5 μm in diameter or long axis). Preferably, these particles (e.g., with NGPs bonded by a polymer, coal tar pitch, or meso-phase pitch) are then subjected to a carbonization treatment to convert the binder material to an amorphous carbon. In many cases, this carbonization also produces micro- or meso-pores in the binder material phase. The resulting nanocomposite particles are now composed of NGPs bonded by a carbon phase. These nanocomposite solid particles are then bonded together with a second binder material (e.g., styrene-butadiene rubber, SBR, poly(tetrafluoroethylene), PTFE, or poly(vinylidene fluoride), PVDF). Surprisingly, these nanocomposites are superior to the already outstanding meso-porous composites invented by us earlier [Ref. 40]. These nanocomposite particles are also superior to carbon black or activated carbon particles when used as an electrode active material for a supercapacitor.
In addition, these nanocomposite particles are superior to meso-carbon micro-beads (MCMBs), conventional fine graphite particles, and conventional graphite spherules when used as an anode active material for a lithium ion battery. For lithium ion battery anode applications, the NGP-containing solid particles do not have to be porous. They can be relatively pore-free solid particles. The presently invented solid nanocomposite particles can be readily mass-produced and are of low cost. Solid particles can be readily made to be smaller than 5 μm if the NGPs chosen are smaller than 2 or 3 μm in size, which are readily available. When used as an anode active material, they exhibit a high reversible capacity, a low irreversible capacity, good compatibility with commonly used electrolytes (no graphite layer exfoliation phenomenon), and a long charge-discharge cycle life.
The present invention provides electrodes for both high-performance lithium ion battery and supercapacitor electrodes. No known prior art graphite or carbon materials are so versatile in electrochemical cell applications.